Characterizing, measuring, and correlating the physical properties of natural gases must take into account this variety of constituents. Phase Behavior of Natural-Gas Reservoirs A widely accepted system for categorizing petroleum reservoir fluids is based on five classes: low-shrinkage crude oils, high-shrinkage volatile oils, retrograde-condensate gases, wet gases, and dry gases. Typical phase diagrams for the gas categories are shown in Figs. McCain Jr. Copyright Pennwell Books, A retrograde-condensate fluid has a phase envelope such that reservoir temperature lies between the critical temperature and the cricondentherm Fig.
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Types of Natural Gas Resources The natural gases can be classified as conventional natural gas, gas in tight sands, gas in tight shales, coal-bed methane, gas in geopressured reservoirs, and gas in gas hydrates.
Conventional natural gas is either associated or nonassociated gas. Associated or dissolved gas is found with crude oil. Dissolved gas is that portion of the gas dissolved in the crude oil and associated gas sometimes called gas-cap gas is free gas in contact with the crude oil.
All crude oil reservoirs contain dissolved gas and may or may not contain associated gas. Nonassociated gas is found in a reservoir that contains a minimal quantity of crude oil. Some gases are called gas condensates or simply condensates.
Although they occur as gases in underground reservoirs, they have a high content of hydrocarbon liquids. On production, they may yield considerable quantities of hydrocarbon liquids. Gases in tight sands are found in many areas that contain formations generally having porosities of 0. At higher gas permeabilities, the formations are generally amenable to conventional fracturing and completion methods.
Of these, eastern Kentucky and western West Virginia are considered the most important. The shale is generally fissile, finely laminated, and varicolored but predominantly black, brown, or greenish-gray.
Core analysis has determined that the shale itself may have up to 12 percent porosity, however, permeability values are commonly less than 1 md. It is thought, therefore, that the majority of production is controlled by naturally occurring fractures and is further influenced by bedding planes and jointing Ikoku Coal-bed methane is the methane gas in minable coal beds with depths less than 3, ft.
Although the estimated size of the resource base seems significant, the recovery of this type of gas may be limited owing to practical constraints. In a rapidly subsiding basin area, clays often seal underlying formations and trap their contained fluids. After further subsidence, the pressure and temperature of the trapped fluids exceed those normally anticipated at reservoir depth. These reservoirs, commonly called geopressured reservoirs, have been found in many parts of the world during the search for oil and gas.
In the United States they are located predominantly both onshore and offshore in a band along the Gulf of Mexico Ikoku In length, the band extends from Florida to Texas; in width, it extends from about miles inland to the edge of the continental shelf. Gas hydrates, discovered in , are snow-like solids in which each water molecule forms hydrogen bonds with the four nearest water molecules to build a crystalline lattice structure that traps gas molecules in its cavities Sloan Gas hydrates contain about times the natural gas by volume under standard conditions.
Because gas hydrate is a highly concentrated form of natural gas and extensive deposits of naturally occurring gas hydrates have been found in various regions of the world, they are considered as a future, unconventional resource of natural gas. Posted by.
Ikoku - Natural Gas Production Engineering